The present invention relates to fluid flow measurement apparatus and more particularly to such apparatus which employs pulsed Doppler ultrasound.
In order to characterize or measure blood flow in a vessel beneath the patient's skin, it has heretofore been proposed to employ ultrasound and to detect or demodulate Doppler shifted components in that portion of the ultrasound energy which is scattered back from the moving blood. The measurement can be performed either invasively, e.g. by means of a catheter, or non-invasively, e.g. by a transducer placed on the patient's skin adjacent a vessel near the surface. Typically, the demodulation of the back scattered energy has involved mixing, e.g. in an analog multiplier circuit, of the returned signal with a reference signal which is coherent with the originally emitted burst. The resulting signal, which represents the relative phase of the received energy, is low pass filtered to remove carrier residue and sum components and sampled at a substantially fixed time interval following each burst to localize velocity sensing to a fixed region. This region is displaced from the transducer by a distance which is proportional to the time interval. The sampling step is followed by low pass filtering to remove frequency components arising from the sampling process itself and to select out the Doppler shifted components [Hartley, C. J. and Cole, J. S.: An ultrasonic pulsed Doppler system for measuring blood flow in small vessels. Journal of Applied Physiology 1974; 37: 626-9] [Baker, D. W.: Pulsed Doppler blood-flow sensing. IEEE Transactions on Sonics and Ultrasonics 1970; SU-17, n3: 170-84]. In order to derive an estimate of the frequency components of the Doppler signal, which is a measure of flow velocities, Doppler signals have been processed by both analog and digital means. Analog processing has typically consisted of zero crossing counting to obtain a mean frequency estimate [Hartley and Cole]. In digital processing, the Doppler signal has been digitized using an analog to digital (A/D) converter and subsequently processed using digital signal processing (DSP) methods such as the Fast Fourier Transform (FFT) to obtain detailed spectral information [Takeda, Y.: Velocity profile measurement by ultrasound Doppler shift method. International Journal of Heat and Fluid Flow 1986; 7, n4: 313-8] [Lutolf, R. M. et al.: Ultrasonic phased-array scanner with digital echo synthesis for Doppler echocardiography. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 1989; 36 n5: 494-506].
Accuracy in the characterization of flow conditions benefit from maximization of two principle qualities of pulsed wave Doppler velocity measurement: 1. spatial resolution, to allow interrogation of many small sample volumes within a flow stream simultaneously (multiple channels), and 2. accurate and detailed estimation of the complex spectrum of frequency components present in the Doppler signal (representing blood cell velocities) from each sample volume. Methods involving analog mixing possess inherent limitations on spatial resolution imposed by the finite duration of the time required to sample the demodulated (phasic) signal (&gt;0.2 us), as well as inherent limitations o location certainty imposed by the slew rate and settling time of the S/H circuit which cause errors related to the derivative of the sampled voltage. Also, to obtain the best spectral estimate requires high-speed A/D conversion followed by cumbersome forms of spectrum analysis, e.g. computation-intensive FFT processing. Multi-channel implementations bearing spectral analysis, though possible, become prohibitively complex [Lutolf et al].
Another demodulation method has involved the comparison of the phase of the entire returned signal from one burst to the [next [Grandchamp, P. A.: Novel pulsed directional Doppler velocimeter: the phase detection profilometer. Proceedings of the European Congress on Ultrasound in Medicine, 2nd; Elsevier Publishing Co., New York, N.Y. 1975; p 137-43]. By providing a continuous measure of frequency shift (phase change per burst) vs. range, this inherently eliminates the sampling requirement, but has the disadvantage of relying on MTI echo cancellation techniques as discussed in the next paragraph. Other methods have involved directly digitizing the raw received signal using an A/D converter with subsequent DSP to demodulate the Doppler components (Gammel, P. M.: Improved ultrasonic detection using the analytic signal magnitude. Ultrasonics 1981; 19, n2: 73-6]. This method requires extremely high speed A/D converters (100 MSPS) with attendant high data rates and speed related design difficulties, and requires specialized, high speed DSP for real-time processing and display, all of which are associated with high cost.
An obstacle to simple acquisition of a pulsed wave Doppler signal is that, because short pulses of ultrasound are transmitted into the tissue, fixed interfaces in the path of the ultrasound beam, e.g. between the blood and the blood vessel, which function as relatively efficient, diffuse reflectors, result in the presence of large, undesired stationary components (fixed echoes) in the received signal. The amplitude of these undesired components is on the order of 40 dB higher than that of the desired Doppler shifted components [McLeod, F. D., et al.; A digital Doppler Velocity profile meter. Rocky Mountain Bioengineering Symposium, 11th, paper; Apr. 15-17, 1974; p 55-60].
The principal mean for reducing the relative level of the stationary components has been the Moving Target Indicator (MTI), adapted from radar applications [Skolnik, M. I.: Introduction to Radar Systems: McGraw-Hill Book Co., Inc., New York, N.Y. 1962; p 113]. This method relies on the use of highly stable, wide bandwidth delay lines which are bulky, expensive, require calibration, and carry the need for complex adjunctive circuitry requiring periodic adjustments. This renders MTI techniques impractical for medical applications of the sort contemplated. Filter [Hartley and Cole] [Peroneau, P. A. and Leger, F.: Doppler ultrasonic pulsed Doppler blood flowmeter. International Conference on Medicine and Biological Engineering, 8th; Jul. 20-25, 1969; session 10-11] and sample/hold (S/H) [Baker, D. W. and Watkins, D. W.: A phase coherent pulse Doppler system for cardiovascular measurements. Annual Conference on Engineering in Medicine and Biology, 20th; Nov. 13-16, 1967; paper 27.2] techniques of stationary echo suppression have also been used. The filtering implementations have had the reputed disadvantage of laborious design and assembly, and certainly limit the response to low velocities. S/H circuitry imposes the limitation mentioned earlier on spatial resolution and location certainty and one S/H is required for each channel in a multi-channel system.
Among the several objects of the present invention may be noted the provision of novel apparatus for characterizing the flow or movement of any fluid material containing suspended solid, liquid or gaseous particles; the provision of such apparatus which can be employed remotely in that the sensing element is not necessarily brought into contact with the fluid under interrogation and does not therefore disturb the movement of that fluid; the provision of such apparatus which generates a signal including frequency components representative of Doppler shifts occasioned by the movement of the fluid; the provision of such apparatus which is highly accurate and reliable; the provision of such apparatus which yields reproducible results; the provision of such apparatus which is of relatively simple and inexpensive construction; the provision of such apparatus which may be relatively simple and inexpensive to expand into a multi-channel implementation. Other objects and features will be in part apparent and in part pointed out hereinafter.